Traditionally, optical networks have comprised multiple point-to point links with little or no intelligence on the optical side. All routing functions in such networks have been carried out on the (digital) electronic side after conversion of the optical signals into digital data in each node in the network.
In the network nodes in a traditional network with optical transfer of data, the optical signals are converted into corresponding data streams where the address information is read and an electronic switch fabric is used to send the data to an output port suitable for its final destination. When the bit-rate increases in the optical channels, this electronic routing technique becomes time and power consuming. Since most data passes through the node en route to further optical nodes, it is necessary to collect data that is to be transmitted 20 through essentially the same optical links and to add some kind of routing label in order to simplify electronic routing in intermediate nodes.
An example of such a system is the so called multi protocol label switching, MPLS, where a central control system manages virtual data paths in the network. With the advent in the late 1990's of dense wavelength division multiplexing, DWDM, where multiple optical wavelengths are transmitted in the same fiber in order to increase the total capacity of the optical point-to-point link, it became natural to also consider using wavelengths to create dynamic optical paths. Thus, some wavelengths could transmit data from node A to node B while some wavelengths could continue to node C without having to be converted into electronic data and inspected in node B. This concept fundamentally decreased the electronic processing requirements and power dissipation in node B. However, the concept required more advanced optical components that could drop a specific optical DWDM channel, and preferably also insert a new channel on the empty wavelength slot. These components are called reconfigurable optical add-drop multiplexers, ROADM, and are based on advanced optical filtering technology and are today to varying extent used in modern optical networks.
The use of ROADMs has increased the flexibility of optical networks, but the cost of ROADM components still hampers large scale use, and today most networks only contain a handful of ROADMs. Another advantage with ROADMs is that their operation is inherently independent of the modulation format of the routed data signals and can thus handle complex modulation formats e.g. QPSK and 16-QAM. Conventional electronic routing schemes can usually not operate independent of optical modulation formats since they need to work on binary data.